1,118 research outputs found
Quantum Control of Qubits and Atomic Motion Using Ultrafast Laser Pulses
Pulsed lasers offer significant advantages over CW lasers in the coherent
control of qubits. Here we review the theoretical and experimental aspects of
controlling the internal and external states of individual trapped atoms with
pulse trains. Two distinct regimes of laser intensity are identified. When the
pulses are sufficiently weak that the Rabi frequency is much smaller
than the trap frequency \otrap, sideband transitions can be addressed and
atom-atom entanglement can be accomplished in much the same way as with CW
lasers. By contrast, if the pulses are very strong (\Omega \gg \otrap),
impulsive spin-dependent kicks can be combined to create entangling gates which
are much faster than a trap period. These fast entangling gates should work
outside of the Lamb-Dicke regime and be insensitive to thermal atomic motion.Comment: 16 pages, 15 figure
Decoherence and thermalization dynamics of a quantum oscillator
We introduce the quantitative measures characterizing the rates of
decoherence and thermalization of quantum systems. We study the time evolution
of these measures in the case of a quantum harmonic oscillator whose relaxation
is described in the framework of the standard master equation, for various
initial states (coherent, `cat', squeezed and number). We establish the
conditions under which the true decoherence measure can be approximated by the
linear entropy . We show that at low temperatures and for
highly excited initial states the decoherence process consists of three
distinct stages with quite different time scales. In particular, the `cat'
states preserve 50% of the initial coherence for a long time interval which
increases logarithmically with increase of the initial energy.Comment: 24 pages, LaTex, 8 ps figures, accepted for publication in J. Opt.
Dynamical Casimir effect for a massless scalar field between two concentric spherical shells
In this work we consider the dynamical Casimir effect for a massless scalar
field -- under Dirichlet boundary conditions -- between two concentric
spherical shells. We obtain a general expression for the average number of
particle creation, for an arbitrary law of radial motion of the spherical
shells, using two distinct methods: by computing the density operator of the
system and by calculating the Bogoliubov coefficients. We apply our general
expression to breathing modes: when only one of the shells oscillates and when
both shells oscillate in or out of phase. We also analyze the number of
particle production and compare it with the results for the case of plane
geometry.Comment: Final version. To apear in Physical Review
Nonadiabatic geometric phase induced by a counterpart of the Stark shift
We analyse the geometric phase due to the Stark shift in a system composed of
a bosonic field, driven by time-dependent linear amplification, interacting
dispersively with a two-level (fermionic) system. We show that a geometric
phase factor in the joint state of the system, which depends on the fermionic
state (resulting form the Stark shift), is introduced by the amplification
process. A clear geometrical interpretation of this phenomenon is provided. We
also show how to measure this effect in an interferometric experiment and to
generate geometric "Schrodinger cat"-like states. Finally, considering the
currently available technology, we discuss a feasible scheme to control and
measure such geometric phases in the context of cavity quantum electrodynamics
Problems with Using Evolutionary Theory in Philosophy
Does science move toward truths? Are present scientific theories (approximately) true? Should we invoke truths to explain the success of science? Do our cognitive faculties track truths? Some philosophers say yes, while others say no, to these questions. Interestingly, both groups use the same scientific theory, viz., evolutionary theory, to defend their positions. I argue that it begs the question for the former group to do so because their positive answers imply that evolutionary theory is warranted, whereas it is self-defeating for the latter group to do so because their negative answers imply that evolutionary theory is unwarranted
In Defense of the Epistemic Imperative
Sample (2015) argues that scientists ought not to believe that their theories are true because they cannot fulfill the epistemic obligation to take the diachronic perspective on their theories. I reply that Sample’s argument imposes an inordinately heavy epistemic obligation on scientists, and that it spells doom not only for scientific theories but also for observational beliefs and philosophical ideas that Samples endorses. I also delineate what I take to be a reasonable epistemic obligation for scientists. In sum, philosophers ought to impose on scientists only an epistemic standard that they are willing to impose on themselves
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